Powders of polyethylene, polypropylene, polybutene and various copolymers are likely to contain solvents during the manufacturing process thereof, so that drying of such powders is generally required to reduce the solvent content thereof.
For example, a slurry polymerization process is known as a method for manufacturing polyethylene which finds wide applications in insulating materials, various containers, pipes, packings, lining materials for industrial apparatus, coating and packaging films and industrial fibers. In this slurry polymerization process, first, ethylene is polymerized in a reactor in the presence of a composite catalyst comprising an alkylaluminum and titanium tetrachloride etc. using a solvent, such as hexane, to obtain a slurry containing a solid polyethylene. Subsequently, the slurry is subjected to a solid liquid separation using a filter to obtain a wet cake of polyethylene powder. Thereafter, the wet cake is dried to obtain a dry polyethylene powder.
The thus obtained polyethylene powder generally contains the solvent, such as hexane, employed in the slurry polymerization, so that drying of the polyethylene powder is required to reduce the solvent content thereof.
The following two methods are known in the art for effecting the drying of the polyethylene powder. In one method, a rotary drying is employed. In particular, the polyethylene powder is dried while being transferred through a rotating cylinder of the rotary dryer. In the other method, use is made of a flash drying apparatus in combination with a fluidized drying apparatus. In particular, first, the polyethylene powder is floated into a high temperature air stream and dried while being transferred by the high temperature air stream (i.e., flash drying). Then, the polyethylene powder having been dried by the flash drying is placed on a porous plate in a fluidized drying apparatus, and hot air is fed from under the porous plate to fluidize and disperse the polyethylene powder so that the polyethylene powder is dried (i.e., fluidized drying).
In the first method, it has advantages in that the operating cost of the rotary dryer is relatively low and the operation thereof is relatively simple. However, the drying of the polyethylene powder by the use of the rotary dryer alone is only effective to reduce the solvent (hexane) content of the polyethylene powder to about 2000 ppm by weight. Since the solvent, such as hexane, contained in the polyethylene powder adversely affects the quality of the polyethylene, it is desired that the solvent content of the polyethylene powder be further reduced. For example, if the solvent content of the polyethylene powder is large, problems are likely to occur with respect to the odor and color of the final product obtained from the polyethylene powder. Further, in the use as a container for food, the elution of the solvent into the food may cause hygienic problems.
On the other hand, in the above-mentioned second drying method, it has disadvantages in that the operating cost of the flash drying apparatus and the fluidized drying apparatus is high and the operation thereof is considerably complicated, although the hexane content of the polyethylene powder can be reduced to about several tens of ppm by weight.
Therefore, any of the conventional drying methods is not satisfactory.